Every time a cell divides, billions of base pairs of information must be accurately copied in the face of an onslaught of DNA damage. Homologous recombination (HR) provides a critical mechanism for tolerating and repairing damaged DNA. Although HR is generally beneficial, misalignments during HR can lead to tumorigenic sequence rearrangements. Despite its fundamental importance, little is known about HR in vivo, primarily because of technical difficulties associated with detecting HR. We have recently created transgenic animals in which fluorescent recombinant cells can be directly detected within intact pancreatic tissue for the first time. This technological breakthrough makes it possible to assess how genetic and environmental factors modulate the accumulation of recombinant cells over time. Pancreatic cancer is the fourth leading cause of cancer death, and risk factors for pancreatic cancer include chronic inflammation and exposure to alkylating agents, both of which are known to induce damaged bases that are repaired by the base excision repair (BER) pathway. In the previous grant cycle, we found that both unrepaired lesions and downstream BER intermediates can induce HR in vitro. Here, we propose to undertake the first ever studies of the influence of BER on HR in pancreatic tissue in vivo. In particular, we will focus on the Aag DNA glycosylase, which removes a broad range of lesions, including many that are created by alkylation damage and inflammatory chemicals. Our hypothesis is that BER substrates and intermediates induce HR, and that the effects of damage are exacerbated by conditions of chronic inflammation or by hormonally-induced mitogenic stimulation. In terms of DNA damage, we will focus on DNA lesions normally repaired by Aag, and on downstream BER intermediates. Our Specific Aims are I) Reveal if Aag substrates or downstream BER intermediates modulate spontaneous or alkylation-induced recombination in vivo;II) Study the potential recombinogenic effects of chronic pancreatic inflammation and hormonally induced cell division in vivo;and III) Determine if inflammation or mitogenic stimulation modulates the effects of DNA damage on HR or alters the extent of clonal expansion of recombinant cells in vivo. The broad long term objectives of this work are to shed light on the molecular and cellular processes that influence a person's susceptibility to spontaneous, environmentally-induced, and cancer therapy-induced DNA sequence rearrangements.